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The scope of this guide is to drawn the guidelines of several different Recommended practice(s) for the

Area Classification of a process plant. The area classification is required for the installation of theelectrical equipment with the related specific protection kind within a process area. The basic

definition, and the following modifications is based mainly on the 1996 NFPA 70, The national

Electrical Code (NEC) and the API 505 Recommended Practice (API RP 505). Once that a location

has been classified, requirements for electrical equipment and associated wiring should bedeterminate from applicable publications (e.g. NFPA 70 and API Recommended Practice 14F (API

RP 14F) and local regulations.

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Purpose:

The final scope of the document is to achieve the classification of both permanently and temporarily

installed electrical equipment. The application is designed in relation to their potential risk of ignition

source in presence of an ignitable mixture of “fuel”, or a flammable/ignitible substance, and Oxygen

(Air) under normal atmospheric conditions.

Reference Atmospheric Conditions

Pressure 101.3 Kpa 14.7 Psia

Temperature 20°C (293.15 K) 68°F

The document provides that is no relevant changes related to the change of the atmospheric

conditions from the reference point. On the basis provided earlier, the guide is developed on the

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recommended practice based on the petroleum facility zones (where ignitable liquids, gases, andvapors are processed, handled and loaded).

References, Codes and Reference Standards:

Actually, there are many Reference standards and industrial codes as reference for the plant areaclassification. Part of them are developed on the same basis, others are very particular and applied in

specific plant type (e.g. Drilling Facilities, Petroleum and petrol chemical plants).

The Hazardous Area Classification presents in this guide is based on the following items as reference:

API:

API RP 505 Recommended Practice for Classification of Locations for Electrical Installation atPetroleum Facilities Classified as Class I, Zone 0, Zone 1 and Zone 2 (2002).

API RP 500 Recommended Practice for Classification of Locations for electrical Installation at

Petroleum Facilities Classified as Class I, Division 1 and Division 2.

IEC:

IEC 60079-10 Electrical Apparatus for explosive gas atmospheres- Part 10: Classifications ofhazardous Area.

IEC 60079-12 Classification of Mixtures of Gases or vapors with air according to their maximum

experimental Gaps (MEGs) and minimum ignition currents ratio (MIC).



IEC 60079-20 Electrical Apparatus for explosive gas atmospheres- Part 20: Data for flammablegases and vapors, relating to the use of electrical apparatus.

NFPA:

NFPA 30: Flammable and Combustible Liquids Code

NFPA 70: National Electrical Code

NFPA 325: Guide to fire Hazard Properties of Flammable Liquids, Gases, and volatile Solids

NFPA 497: Recommended practice for the Classification of Flammable Liquids, Gases or Vapors andof Hazardous (classified) Locations for Electrical Installations in Chemical Process Areas.

Basic Definitions:

The following list of definition is based on the reference codes and practice guideline listed before. Thereference standard is assigned to each definition.

Boiling Point – The temperature of a liquid boiling at the reference atmospheric conditions. (IEC 79-

10, Mod.)

Area Classification – See Further paragraph named (“Area Classification and Definition”).

Class I, Zone 0 – See Further paragraph named (“Area Classification and Definition”).



Combustible Liquid(s) – See Flammable Liquid(s) definition.

Enclosed Area – A three-dimensional space enclose by more than two-third (2/3) of the possibleprojected plane surface area and of sufficient size to allow the entry of personnel. For a common

building, this would required two-third (2/3) of the walls, ceiling, and/or floor be present.

Explosive gas atmosphere – A mixture with air, under the reference atmospheric conditions, of a



flammable material in the form of gas or vapor which, after ignition, combustion spreads throughout

the unconsumed mixture. (API 505-3.2.20)

Flammable – Capable of an easy ignition, burning intensely or spreading flame rapidly.

Flammable (Explosive) limit(s) – The lower (LFL) and upper (UFL) percentages by volume of

concentration of gas in gas-air mixture that will form an ignitible mixture. (NPFA 325)

Flammable Liquid(s) – See Further paragraph named (“Flammable liquid Classification”).

Flash Point – The minimum temperature of a liquid at which sufficient vapor is give off to form anignitible mixture with air, near the surface of the liquid, or within the vessel used, as determinate by

the test procedure and apparatus specified in NFPA 30.

Grade of Release – There are three basic grade of release, as listed below, in order of decreasinglikelihood of the explosive gas atmosphere being present.(1)

1- Continuous

2- Primary

3- Secondary

Other grades of release may be possible by combination of the basic ones listed.(IEC 79-10, Mod.)

(1) It is important to underline that there isn’t any relationship with the type of release discussed

earlier like” puff” and “plume”.

Grade of Release: Continuous – See Further paragraph named (“Area Classification and

Definition”).

Grade of Release: Primary – See Further paragraph named (“Area Classification and Definition”).

Grade of Release: Secondary – See Further paragraph named (“Area Classification and Definition”).

Gas Group(s) – For the Classification, the ignitible gases or vapors are classified in several different

groups. The subdivision of the gases is related to the gases physical and chemical properties.

Hazardous (classified) Location(s) – A location where fire and explosion hazards may exist due toflammable gases or vapors, flammable liquids, combustible dusts, or ignitible fibers of flyings. (API

505-3.2.10.5)

Heavier-than-air Gases of Vapors – Formally those gases of vapors with a relative density above 1.2

as to be regarded as Heavier-than-air gases. (IEC 79-10, Mod.)

Highly volatile liquid(s) (HVL) – See Further paragraph named (“Flammable liquidClassification”).

Ignitible (Flammable) Mixture – A gas-air mixture that is capable of being ignited by an open

flame, electric arc or spark, or device operating above the ignition temperature of the gas-airmixture. (See “Flammable (Explosive) Limits”) (API 505-3.2.32)



Ignition (Auto ignition) Temperature (AIT) – The lowest temperature of a heated surface at which,under specific conditions, the ignition of a flammable substance, or mixture in the form of gas or

vapor will occur. (IEC 79-10, Mod.)

Lighter-than-air Gases or Vapors – Formally those gases or vapor with a relative density below 0.8as to be regarded as Lighter-than-air substances. (IEC 79-10, Mod.)

Maximum Experimental Safe Gap (MESG) – The maximum gap of the joint between the two parts

of the interior chamber of a test apparatus that, when the internal mixture is ignited and underspecific conditions, prevents the ignition of the external gas mixture by propagating through a 25 mm

(984 mils) long joint, for all concentrations of the tested gas or vapor in air. (API 505-3.2.38)

Minimum Ignition Current (MIC) – The minimum current that, in a specified spark test apparatusand under specific condition, is capable of igniting the most easily ignitible mixture. (API 505-3.2.39)

Minimum Ignition Current Ratio (MIC Ratio) – The minimum energy required from a capacitive

spark discharge to ignite the most easily ignitible mixture of a gas or vapor divided by the minimumcurrent required from and inductive spark discharge to ignite methane under the same test conditions.

(NFPA 497)

Normal Operation(s) – The situation when the equipment is operating within its design parameters.(IEC 79-10, Mod.)

Protected Fire Vessel – Any fired vessel that is provided with equipment (such flame arresters, stacktemperature shutdown, forced draft burners, with safety controls, and spark arresters) designed to

eliminate the air intake and exhaust as sources of ignition. (API 505-3.2.48)

Release, Source of – A point or location from which a flammable gas, vapor or liquid may be releasedinto the atmosphere such that an ignitible gas atmosphere could be formed. (IEV 426-03.06, Mod.)

Release Rate – The quantity of flammable gas or vapor emitted per unit time from the source of

release. (IEC 79-10, Mod.)

Vapor Pressure - The pressure exerted when a solid or liquid is in equilibrium with its own vapor. It

is a substance properties linked to the environment condition and determinate by ASTM D 323-82.

(IEC 79-10, Mod.)

Vapor-tight Barrier – Is a wall, or barrier that will not allow the passage of significant quantities of

gas or vapor at atmospheric pressure. (API 505-3.2.54)

Ventilation – Natural or artificial movement of air and its replacement with “fresh air”.

Ventilation, Adequate – Ventilation that is sufficient to prevent the accumulation of enough

quantities of an ignitible mixture into a specific location.

Volatile Flammable Liquid – A flammable liquid whose temperature is above its flash point, or a

Class II combustible liquid having a vapor pressure not exceeding 276 Kpa (40 Psia) at

37.8°C(100°F) whose temperature is above its flash point. (API 505-3.2.58)

Basic Condition for Fire(s) and Explosion(s):



A

T

s discussed earlier, to occur, a fire and/or and explosion needs three basic elements, without any of

them, or specific conditions for each of them, the event cannot occur. The three main elements are:

(1) A fuel, not necessary an common combustible (e.g. Dust, or Mill Dust), (2) a combustible (e.g. Airor Oxygen). (3) An igniter source with enough energy to ignite the flammable mixture (e.g. Electrical

equipment, free flames, or hot surfaces). Other than the presence of each of these elements, there are

two additional conditions needed to obtain a fire or an explosion: (4) The concentration of the fuelwithin the mixture must be between its own Upper and Lower Flammable Limit. (5) The three basic

elements must be in same location, or they must have a position that allows them to complete their

own role.

In classifying a particular location, the likelihood of the presence of a flammable gases or vapor is a

significant factor in determinate the zone classification (See Further paragraph named “AreaClassification and Definition”). Otherwise a distinction must be made: the presence of the flammable

mixture could be distinguished between “normal conditions” and “extraordinary condition”. The

term “extraordinary condition” doesn’t mean only a catastrophic event like a violent breakage of an

item or similar, but also an ordinary maintenance operation. There is obviously an objection: If an

item, or a location, needs a frequent maintenance, the act itself will go under the “normal condition”.

(API 505.4.2 refers to these condition adopting the phrase “Normal and Abnormal Condition”).

As said, the mixture, to occur into an explosion and/or a fire, must have a concentration within its

range of flammability. It is quite important to know or to reach an approximation of the quantities of

flammable mixture are present inside the different location, to determinate the extension of the area.

As more the released quantities are high, as more the area affected by the hazard is wide.

Another relevant parameter to take into account is the ventilation. The ventilation of a specific

location can reduce sensibly the hazard connected to a ignitible substance release, even in major case.

A good ventilation, natural and/or artificial), especially inside enclosed location, is the fist measure toadopt to reduce the risk of Fires.

Especially for preliminary studies, even before the engineering starts, where the knowledge of the

plant and the area is almost unknown, found even and approximate form of these parameters

(Likelihood, Concentration, and Ventilation of a specific area) could be really hard, and in the best

case the approximation is totally aloof from reality. In fact, the hazardous area classification is

commonly made during the entire development of the plant, from the first plot plan revised by the

process company to the final general plot plan of the engineering phase, reviewing continuously thedata and the area classification.

Safety Principles:

he area classification must be carried out when the initial process and instrument line diagram(P&ID) are available and will be frequently updated till the start up of the plant. Even during the life

of the plant, the classification must be periodically updated, to take into account potential changes to

the original plat, or new release source(s).The hazardous area classification, especially its first

application, during the process design, must be mainly affected from the inhertly design principles. To

start the area classification, it must be identified those area containing hazardous substance,

examining those area and identifying the potential sources or points of leakage. The first question the

operator should ask himself is “Can these sources be replaced?”, avoiding source or any release point



it could be the best strategy to adopt, especially when the engineering of the plant even had started.

“If the source cannot be avoided, the rate of the release, or its frequency, or its likelihood can be

reduced?” the hazard area related to a release is as low as the quantity released, the time of the

release and its likelihood is low.

Flammable liquid(s) Classification:

Substances handled by any process facilities include flammable and combustible liquids, flammable

highly volatile liquids (HVLs) and flammable gases and vapors. When classifying locations for

electrical installations, the appropriate class and group(s) should be determinate for all flammable

substances. Each group, and class, is related to the physical and chemical features of the substance.

Refer to NFPA 325 or 497 for the properties of specific flammable liquids, gases, vapors and volatile

solids. The volatility of flammable and combustible liquids is defined in NFPA 30.

Flammable (Class I) Liquids, such a gasoline, are defined as any liquid having a closed-cup flash point

below the threshold of 37.8°C (100°F) and a vapor pressure not exceeding 276 Kpa (14 Psia).

Combustible (Class II and III) liquids are defined as liquids having a closed-cup flash point at or

above the threshold of 37.8°C (100°F) and below 60°C (140°F). Class III liquids are those liquids

having a closed-cup flash point above the threshold of 60°C (140°F).

Flammable and Combustible Liquid(s)

ClassI II III

Flash Point

Range(°C) < 37.8 37.8 ÷ 60

> 60

Vapor

Pressure(Kpa) > 276 N.a. N.a.

A common error is to confuse these classes and liquids group with the Classes of The National

Electrical Code, they are not synonymous.(NFPA 70).



Closed-Cup Flash Point and Auto ignition temperature:

Another spread error is to confuse the Flash point with the auto ignition temperature.

“Flash Point – The minimum temperature of a liquid at which sufficient vapor is give off to form an

ignitible mixture with air, near the surface of the liquid, or within the vessel used, as determinate by

the test procedure and apparatus specified in NFPA 30.”

The auto-ignition temperature (AIT) instead, is the temperature, above the flash point, which allow

the ignition of the flammable mixture without any ignition source. Adopting the definition of the IEC:

“Ignition (Auto ignition) Temperature (AIT) – The lowest temperature of a heated surface atwhich, under specific conditions, the ignition of a flammable substance, or mixture in the form of gas

or vapor will occur. (IEC 79-10, Mod.)”

The term “Closed-Cup” behind the flash point definition is referred to the standard apparatus

adopted for its assessment. There are two basic types of flash point measurement: open cup and

closed cup.

In open cup devices the sample is contained in an open cup which is heated, and at intervals a flame is

brought over the surface. The measured flash point will actually vary with the height of the flame

above the liquid surface, and at sufficient height the measured flash point temperature will coincide

with the fire point. The best known example is the Cleveland open cup (COC).

There are two types of closed cup testers: non-equilibrium, such as Pensky-Martens where the

vapours above the liquid are not in temperature equilibrium with the liquid, and equilibrium, such asSmall Scale (commonly known as Setaflash) where the vapours are deemed to be in temperature

equilibrium with the liquid. In both these types the cups are sealed with a lid through which the

ignition source can be introduced. Closed cup testers normally give lower values for the flash point

than open cup (typically 5-10 °C) and are a better approximation to the temperature at which the

vapor pressure reaches the lower flammable limit (LFL).

The flash point is an empirical measurement rather than a fundamental physical parameter. Themeasured value will vary with equipment and test protocol variations, including temperature ramp

rate (in automated testers), time allowed for the sample to equilibrate, sample volume and whether

the sample is stirred.

Methods for determining the flash point of a liquid are specified in many standards. For example,

testing by the Pensky-Martens closed cup method is detailed in ASTM D93, IP34, ISO 2719, DIN

51758, JIS K2265 and AFNOR M07-019. Determination of flash point by the Small Scale closed cup

method is detailed in ASTM D3828 and D3278, EN ISO 3679 and 3680, and IP 523 and 524.

Flash point, and auto-ignition temperature examples are reported above.For further information

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about Flash point test, and data refer to NFPA 30.

Flammable Highly Volatile Liquid(s):

The HVLs such butane, propane, propylene, etc, are those flammable liquids of the first class, with a

vapor pressure exceeding 276 Kpa (40 Psia) at 37.8 °C ( 100°F).

Those liquids, have a high volatility and a low Flash point, allowing them to produce a large volume

of ignitible mixture. All the precautions must be taken if handled inside the process plant, and they

must be treated with ad over conservatively method. Usually the mixture generated has a density

lower than air, allowing quick movement and wide distance covered. Those attitudes are the mostdangerous and result into a wide process area designed with strictly safety applications.

Class I Liquids:

Class I liquids, with the lowest Flash Point range, are usually handled at temperature above the safety

range, consequently can produce a flammable atmosphere. Especially when released into the

atmosphere, may produce a large volume of vapors (rarely gases), especially near the source of

release. Even if the Class I liquids are generally related to the highest hazard, the less volatile liquids

of this class release vapors slowly and the ignition is possible only if the igniter (or the ignition source)is placed near the liquid pool (See.”Pool fire”).

Class II Liquids:

With class II liquids, the likelihood of ignition is much lower than Class I liquids, due to

their higher flash point, only few of them are usually handled at temperature above their flashpoint.

The ignition of a liquid of the Class II usually is taken into account when exposed to a fire, in this case

the risk analysis is behind the common threshold of “One emergency only“.

Even if they generate vapors, them usually are quite heavy and they cannot move to far from the

source. The transition state of the ignitible mixture from Heavier-than-air to lighter-than-air is quite

slow.

API RP 505 refers to the Class II liquids as “Do not produce vapors of sufficient quantity to be

considered for electrical classification”- (5.2.3.2)

Class III Liquids:

Similar to the Class II liquids, the Class III has a lower likelihood to produce an ignitible atmosphere

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Flash Point

°C

AIT

°C

Benzene12 560

Ethyl Alcohol55 365

Gasoline-45 280

N-Butane-76 405

N-Hexane-54 225

I-Butane-117 462

due to their highest flash point and lowest volatility. If heated beyond its flash point, a Class III liquid

produce a small amount of vapor, localized near the point of release.

A partial list of flammable liquids, with the related Class, groups and category are reported into the

following pages (NFPA 497 – 1997).

Crude Oils:

A specific classification for the crude oils is impossible, its composition is too complex and diversified

that even its common physical properties are generic. However crude oil is generally classified as

Class I flammable liquid and its flash point range, generally accepted, is -6.7 – 32.2 °C (20-90°F).

Gas Groups:

As done for the hazardous liquids, also the gases are divided into several different groups and

subclasses. The first subdivision is the classification into two groups: (1) Group I – Underground

Gases and (2) Group II – Aboveground Gases. The first class, is referred to describe atmospheres

containing firedamp (a mixture of gases, composed mostly by methane, found usually in mines). For

this reason the group I of flammable gases are not used during the Hazardous area classification. Thesecond group is use d to describe all the gases found above ground and is subdivided into IIC; IIB and

IIA according to the nature of the gas or vapor. The last three categories of gases are defined by two

physical properties of the substance: Maximum experimental allowable gap(s) (MEGS) and the

Minimum Ignition Current ratio.

Maximum Experimental Allowable Gap:

The MESG is defined as follow:

“Maximum Experimental Safe Gap (MESG) – The

maximum gap of the joint between the two parts of

the interior chamber of a test apparatus that, when

the internal mixture is ignited and under specificconditions, prevents the ignition of the external gas

mixture by propagating through a 25 mm (984 mils)

long joint, for all concentrations of the tested gas or

vapor in air. (API 505-3.2.38)”

The maximum experimental safe gap of flammable

gases and vapors is the lowest value of the safe gap

measured according to IEC 60079-1-1 by varying thecomposition of the mixture (“flame propagation in

the most ignitibie mixture”).The safe gap is the gap

width at which in the case of a given mixture

composition, a flashback just fails to occur. The test

procedure and its parameters are fully describe in



IEC 60079-1-1.

Minimum Ignition Current Ratio (MIC Ratio):

The MIC RATIO is defined as follow:

“Minimum Ignition Current Ratio (MIC Ratio) – The minimum energy required from a capacitive

spark discharge to ignite the most easily ignitible mixture of a gas or vapor divided by the minimumcurrent required from and inductive spark discharge to ignite methane under the same test conditions.

(NFPA 497)” For further information about the MIC Ratio refer to IEC 60079-3.

Gas Group IIA:

Atmospheres containing Acetone, Ammonia, Ethyl Alchol, Gasoline, Methane, propane or flammable

gas, flammable liquid produced vapor, or combustible liquid mixed with air that may burn or explodehaving either a MESG value greater than 0.90 mm or a minimum igniting current ratio (MIC Ratio)

greater than 0.80 (NFPA 497 or IEC 60079-10)

Gas Group IIB:

Atmospheres containing acetaldehyde, hydrogen, of flammable gas, flammable liquid produced vapor,

or combustible liquid produced vapor mixed with air that may burn or explode having either amaximum experimental safe gape (MESG) greater than to 0.50 mm (20 mils) and less than or equal to

0.90 mm (35 mils) or a minimum igniting current ratio (MIC Ratio) greater than 0.45 and less than orequal to 0.80 (NFPA 497).

Gas Group IIC:

Atmospheres containing acetylene, hydrogen, of flammable gas, flammable liquid produced vapor, or

combustible liquid produced vapor mixed with air that may burn or explode having either a maximumexperimental safe gape (MESG) greater than or equal to 0.50 mm (20 mils) or a minimum ignitingcurrent ratio (MIC Ratio) less than 0.45 (NFPA 497).

Group II

IIA IIB IIC

MESG (mm) > 0.90 0.50 < < 0.80 < 0.50

MIC Ratio > 0.800.45 < < 0.80

<0.45



For the most gases and vapours it is sufficient to make only one determination between the MESGand MIC Ratio to relate the gas to its own group. There are some cases which both determination are

required:

a) When MIC Ratio is within the range 0.8 – 0.9.

b) When MIC Ratio is within the range 0.45 – 0.5.

c) When MESG is within the range 0.5 – 0.55

In those cases, the second parameter will classify the gas and its group. The IEC provides a complete

list of common substances with their own group and properties inside the IEC 600 79-12 (1978).

Hazardous Area Classification

General Criteria:

The choice of classify an area is based on the assumption that flammable gas(es), vapor(s) or ignitible

liquid(s) may be present. The choice starts from two basic assumption:

a) – There is an Hazardous Substance.

b) – There is a potential Source of Release.

Possible source of release include: vents, flanges, control valves, pump and compressor sealing,fittings, and floating roof seals. It is obvious that the presence of those elements inside a process plat

is so spread and wide that usually not each of them must be considered during the area classificationas a potential source of ignitible, o generally, hazardous substances. The following paragraphs willexplain which of them are significant and have a relevance inside the classification study. Once that

the substances are identified and the potential source localized, before the area classification, otherfactors must be taken in account like the grade of release, grade of ventilation and the topography of

the plant.

Commonly, the grade or release is distinguished into three different category:

- Continuous.

- Primary.

- Secondary

Each of these categories leads naturally to one specific kind of location, but there is no firm rules (seeClassification Precautions 9.10.VI). However the grade of release is strictly connected to the time ofrelease:

Grade ofRelease

Flammable Mixture time of

release



Continuous 1000 or more Hours/year

Primary 10 < Hours/year < 1000

Secondary 10 < Hours/year

Once that the type of release has been indentified, the ventilation assessment of the specific area isrequired. The ventilation assessment is the last step to classify a specific area. To evaluate the

extension of a specific area a great number of information must be gathered, especially:

- Volatility of the flammable liquid

- Flash point

- Liquid Temperature at release

- LFL and UFL

- Vapor and liquid density

- Geometry of the source of release

- Quantity Released

- Potential Release Rate

- Concentration and starting conditions

- Ventilation

- Process Plant Topography

- Climatic Conditions

Looking at the great number of information required, it is simply notice that have an accurate

estimate of the hazardous area is really hard to get. Many codes and Recommended practice suggestseveral different examples and suggestions to achieve a correct safety levels, not underestimating the

potential hazard (API RP 505 or NFPA 497).

Area Classification and Definition:

The area classification, especially the area definition, is the same for several different codes andstandards like: API; IEC. NFPA. National Electrical Code (NFPA 70), etc.

The following paragraph is developed on the API RP 505 basis, and the area definition are reported



from it.

Class I, Zone 0:

Class I, Zone 0 presents the following features:

(1) Is a location in which a presence of ignitible concentration, or flammable gases, or combustible

liquids is continuous;

Or

(2) In which an ignitible concentration, or flammable gases, or combustible liquids is present for a

long time.

This locations usually includes locations inside vented tanks or vessels containing volatile flammableliquids; the volume between the inner and the outer roof section of a floating roof tank containing

volatile flammable liquids; inside open vessels, tanks and pits; and inside inadequately ventilatedenclosures containing normally venting instruments utilizing or analyzing flammable fluids andventing the inside of enclosures.

Class I, Zone 1:


(1) Is a location in which a presence of ignitible concentration, or flammable gases, or combustibleliquids are likely to exist under normal operating conditions;

Or

(2) - in which an ignitible concentration, or flammable gases, or combustible liquids may exists

frequently because of repair or repair maintenance operations or because of leakage;

Or

(3) In which equipment is operated or process carried on, of such nature that equipment breakdown

or faulty operations could result in the release of ignitible concentrations of flammable gases orvapors and also cause simultaneous failure of electrical equipment in a mode to cause the electrical

equipment to become source of ignition.

Or

(4) Is adjacent to a Class I, Zone 0 location which ignitible concentrations of vapors could be

communicated, unless communication is prevented by adequate positive pressure ventilation formsources of clean air and effective safeguards against ventilation failure has been taken.



This classification usually includes locations where flammable liquids or liquefied flammable gases aretransferred from one container to another; inadequately ventilated pump rooms for flammable gas or

for volatile flammable liquids; the interior of refrigerators and freezers in which volatile flammablematerials are stored in the open, lightly stoppered, or easily ruptured containers; and other locations

where ignitible concentrations of flammable vapors and gases are likely to occur in the course ofnormal operation but not classified as zone 0.

Class I, Zone 2:


(1) Is a location in which a presence of ignitible concentration, or flammable gases, or combustibleliquids are not likely to occur under normal operating conditions and if the do occur will exists for ashort period of time;

Or

(2) - in which an ignitible concentration, or flammable gases, or combustible liquids are handled

,loaded, unloaded, processed, or used, but in which the liquids, gases, or vapors normally are confinedwithin closed containers of closed system from which they can escape ONLY as a result of accidental

rupture or breakdown of the containers or system, or as result of the abnormal operation of theequipment with which the substances are handled or processed;

Or

(3) In which ignitible concentrations of flammable gases or vapors normally are prevented bypositive mechanical ventilation, but which may become hazardous as a result of failure or abnormal

operation of the ventilation equipment;

Or

(4) Is adjacent to a Class I, Zone 1 location which ignitible concentrations of vapors could be

communicated, unless communication is prevented by adequate positive pressure ventilation formsources of clean air and effective safeguards against ventilation failure has been taken.

The zone 2 classification usually includes locations where volatile liquids, or flammable gases orvapors are used, but that would become hazardous only in case of an accident or of some unusual

operating condition.

Zone Considerations:

The word “Normal” are commonly used inside the zone definitions, but is not synonymous of“everything is working properly”. For instance, a process may be so sensitive to control that the

activation of safety valves or other pressure relief equipment, o generally safety equipment, isfrequently, and that should be considered “normal”.



The frequency of the maintenance or repair is relevant to define the “normal” and “abnormal”

situation: if the maintenance is required frequently the operations are to evaluate as “Normal”,however if the maintenance frequency is low, its operations are included into the abnormal category.

Zone 1 location are usually placed as border line of each Zone 0 location; to separate Zone 0 and

Zone 2 locations (Called “Transition zone(s)”). To prevent the extension of an Zone 1 location asborder of the Zone 0 location, it can be used a vapor-tight barrier (see “definitions”) to avoid the

spreading of the hazardous substances.

Class I, Zone 2 locations are those area when abnormal conditions will generate a release. Forinstance, a leakage from a sealing is quite rare, and can be considered abnormal. Also the

simultaneous rupture of the equipment and the electrical installation is quite rare.

Zone 2 location are usually placed as border line of each Zone 1 location (Called “Transition

zone(s)”). To prevent the extension of an Zone 2 location as border of the Zone 1 location, it can beused a vapor-tight barrier (see “definitions”) to avoid the spreading of the hazardous substances.

Classification Precautions:

There is a natural relationship between the grade of release and the type of zone.

Zone Flammable

Mixturepresence

(Hour/year) Grade of Release 0

1000 or more

Continuous1

10 ÷ 1000

Primary2

1 ÷ 10

SecondaryUnclass.

Less than 1

The grade of release, and the kind of zone are not synonymous, in fact a continuous grade of releasenaturally leads to a constant presence of the flammable mixture in the area. But a specific assessment

should be taken for each specific place.

Unclassified Zones:



Many process plant locations may remains unclassified at the end of the classification, why? Because the

experience has shown that, regardless the grade of ventilation of the location, the likelihood of an accidentalleakage and the consequent release of hazardous material are quite rare.

In those area category are included:

1) Locations where flammable substances are contained in all-welded closed piping systems withoutvalves, flanges or similar device.

2) Locations where flammable substances are contained in continuous metallic tubing without valves,flanges or similar device.

3) Adequate ventilated location surrounded by process items with continuous flame sources (e.g. flare tips)

4) Locations with any non-electric ignition sources (flare tips, free flame, etc) .

Ventilation:

In the major part of the cases, the ventilation is one of the most important parameter for the classification ofan area. Eventually gas or vapors leaked to the atmosphere can be diluted by dispersion or diffusion into theair dropping their concentration below the LFL concentration threshold. The ventilation, natural or artificial,

leads to the continuous replacement of the air of a specific zone with “fresh air”. The fresh air must come froman unclassified or Zone 2 location to be efficient and positive.

Suitable ventilation rates can also avoid persistence of an explosive gas atmosphere, thus influencing the typeof zone (IEC 600 79-10. Mod.).

An “Adequate Ventilation” is defined as ventilation (natural or artificial) that is sufficient to prevent theaccumulation of a concentration of flammable substances over the twenty-five percent (25%) of theconcentration LFL threshold. Refer to NFPA 30 for additional details.

Ventilation Assessment:

The assessment of the ventilation system(s) can be done evaluating the hypothetical volume released and the

time of residency of the same. The following analytical method is subject to several limitations, but adoptingsuitable safety factors, the error resulted is on the side of safety.

To start the assessment, it must be know the maximum release rate of the substance (G)

The first step is to evaluate the minimum volumetric flow of fresh air (Vair) needed to dilute the hazardous

mixture:

Where:

k = 0.25 (Safety Factor) for primary and continuous grade of release, 0.50 for secondary grade of release.

LFL = Lower Flammable limit of the mixture

T = Ambient Temperature (in Kelvins)

Generally LFL are expressed as (Vol%), to convert it to (Kg/m3) use the following formula:



Where:

MW = Molecular Mass of the substance (Kg/mol).

Now we need to set the number of cycles of change, for the air, per time unit C (S-1), and the potential

hazardous volume around the source can be estimated:

Where:

Vz = Potential Hazardous Volume near the release source (m3).

f = is an additional safety (and quality) factor equal to 1 ÷ 5 that denotes the quality of the ventilation system.

The greatest value (5) means that the air flow is not fresh or is impeded.

Once that Vz is estimated, it identify the volume within the ignitible mixture concentration is at least 25 or 50

% (related to the k value adopted) of its own LFL.

For an Enclosed Area (which 2/3 of the outside surface of the volume is covered) the number of cycles ofchanges of air per unit of time is defined by the following formula:

Vtot = is the total volume of the air flow rate.

Vo = is the total volume of the enclosed area.

In an open air situation, even with the lowest wind speed, the number of cycles of change air are high.

Usually is adopted a C equal to 0.03 s-1 (related to a wind speed of 0.5 m/s). However, this method is

quite conservative, over-sizing the hazardous area.

Once that the hazardous volume has been defined, the next step to estimate the grade of ventilation isto achieve the Persistence Time (t) of the flammable mixture. The time required for the average

concentration to fall from the starting value of Xo to the LFL multiplied by k after the release has

stopped can be estimated from:

t is estimated in the same time unit of C. A particular attention must be adopted for the Xo value:

inside the hazardous volume, the concentration of the flammable substance vary sensibly between the100% (in the whole area near the source of release) and the 25% of the LFL. However the proper

value of Xo should be estimated for each case.

The residence time estimated, isn’t a quantitative value for the area classification, it gives additional

information about the abnormal process, and must be compared to the time scale of the specificprocess.An acceptable time of dispersion depends by the time and frequency of the release.

The volume Vz can be used to provide a means of rating the ventilation as: High, Medium, or low. The

persistence time can be used to provide a means of rating the ventilation required to comply with the

definition of Area 0, 1 or 2.

In particular ventilation can be rated as:



- High – when Vz is small, almost negligible.

- Medium – when the Vz volume can be controlled.

- Low – when the Vz volume cannot be controlled.

The code IEC 600 79-10 relate the value of Vz to the rate of the ventilation:

Ventilation Rate

High Medium Low(2)

Vz (m3)< 0.1 0.1 < Vz < Vo

> Vo

2) Low Ventilation cannot occur in open space cases.

With High Ventilation Grade, the ventilation is so efficient that the Hazardous volume can be

considered negligible and the related area my remain unclassified. An high grade of Ventilation can beassigned only in cases of Artificial Ventilation, into small enclosed area.

Another relevant parameter to consider is the “Ventilation availability”. The availability needs to be

taken into account during the area classification the type of zone. The level of availability are thefollowing:

A – Good – Ventilation is present virtually continuously.

B – Fair – Ventilation is present during normal operation.

C – Poor – Doesn’t met the feature of “good” and “Fair”, but discontinuities are not to be expectedto occur for long periods of time.

In open space cases, the wind action is included under the Good category even at the lowest speed(0.5 m/s). The effect of the ventilation to the area classification is summarized in the following table.

Grade of

Release Ventilation

Degree



High Medium Low

Availability

Good Fair Poor Good Fair Poor Good, Fair, Low

Continuous (Zone 0 NE) (Zone 0 NE) (Zone 0 NE) Zone 0 Zone 0

+2

Zone 0

+1

Zone 0

Primary (Zone 1 NE) (Zone 1 NE) (Zone 1 NE) Zone 1 Zone 1+2

Zone 1+2

Zone 1 or 0

Secondary (Zone 2 NE) (Zone 2 NE) Zone 2 Zone 2 Zone 1 Zone 2 Zone 1 and even0

The Zone definition NE indicates a theoretical zone that would be negligible extent under normal

operating conditions. The symbol “+” adopted means “Surrounded by”.

Multiple source:

An enclosed area, o in general, a specific zone could have several different release sources. How can I

assess the ventilation? IEC 600 79-10 presents a simple method to evaluate the ventilation in amulti-source zone. The value of to adopt, during the calculation, depends from the grade of release.

The following table shows the value to adopt for each grade of release category.

Grade of Release Action to be taken with

Continuously Summate all values for each source and use the result.

Primary In accordance to the following table, summate the requisite number of the

largest value of and apply the resulting total.

Secondary Use only the largest volume of the sources of the same area.

NOTE: Differing grade of release are not required to be summated.

Number of primarygrade releases

Number of primarygrades releases to beused in accordance

with the previoustable



1 1

2 2

3 to 5 3

6 to 9 4

10 to 13 5

14 to 18 6

19 to 23 7

24 to 27 8

28 to 33 9

34 to 39 10

40 to 45 11

46 to 51 12

9.11.III – Calculation Examples:

Example N°1:

Characteristics of the

release

Flammable Material Propane (gas)

Source of Release Can-filling nozzle

LEL



VentilationCharacteristics

Indoor situation

Number of airchanges, C

20 per hour

Quality factor, f 1

Ambient

Temperature,T

35°C (308 K)

Ventilation

Characteristics

Indoor situation

Number of air

changes, C

15 per hour

Quality factor, f 1

AmbientTemperature, T

20°C (293 K)

0.0039 [Kg/m3]

Grade of release Primary

Safety Factor, k 0.25

Release Rate, G 0.005 [kg/s]

Minimum volumetric flow rate of fresh air:

Estimation of Hypothetical Volume Vz:

Time of persistence t:

Conclusion: The hypothetical volume Vz is significant but it can be controlled. The degree of

ventilation Is considered as medium with regard to the source based on this criterion. With apersistence time of 0.26 h, the concept of Class I, Zone 1 may not be met if the operation is repeatedfrequently.

Example N°2:

Characteristics of therelease

Flammable Material Ammonia (gas)

Source of Release Evaporator Valve

LEL 0.105 [Kg/m3]

Grade of release Secondary


Release Rate, G [kg/s]




Conclusion: The hypothetical volume Vz isnegligible. However the items adjacent to the valve should be classified as Zone 2.

Example N°3:

Characteristics of therelease

Flammable Material Toluen (vapor)

Source of Release Flange



Ventilation

Characteristics

Indoor situation

Number of air

changes, C

once per hour

Quality factor, f 5

AmbientTemperature,T

20°C (293 K)

T

LEL 0.046 [Kg/m3]

Grade of release Continuos


Release Rate, G [kg/s]




The time of persistence cannot be estimate for continuous release. Conclusion: The hypotheticalvolume Vz is negligible. However the items adjacent to the flange should be classified as Zone 2.

Adjacent Area:

For the area adjacent to the classified one, particular attention must be adopted. There three differentcases:

1) – A non closed adequately ventilated area that is adjacent to a classified area, and that is NOTseparated from the classified area by vapor tight-barrier, should be classified to the extent designated.

2) – A enclosed area that is adjacent to a classified area, that is separated from the classified area

by a vapor tight-barrier, is unclassified, considering only the external sources.

3) – An enclosed area that is adjacent to a classified area, and that is NOT separated from theclassified area by the vapor tight-barrier, should be classified the same as the highest classification

included. (API RP 505).

Extent of the hazardous area:

he extent of a particular zone, is function of several factors and parameters. The API RP 505 suggests theextent of area generated by different potential release sources (e.g. Vents, Rupture disk, etc.).

Refer to API RP 505, NFPA 497 for the single source. In those cases, the extent of the single area isdetermined only by the location of potential sources of release of flammable liquids, gases or vapors, and not



by the location of the igniters (electrical and non-electrical).

To estimate the extent of the specific area, especially for particular cases not revised by the common

standards, there are many dispersion model, even software suitable.

An alternative method for the area Classification:

API RP 505 suggests an alternative method for the area classification. The method is based on theconcept of “Point Source”, involving the creation of specific boundaries for each individual source.

At the end of the specific classification, an extended area classification is made by the composition ofthe adjacent different area.

The extent of the area is related to: 1) the grade and velocity of the release 2) and the volatility of thesingle substances. ( the volatility of a mixture is equal to the highest volatility of its components).

The “Hazard Radius” is a direct function of volatility and rate of release, that means that as one of

the last parameter is reduced, the extent of the hazardous area tend to be reduced.

The rate or the speed of the release (the quantity released) is divided into three different categories:High, Medium and Low.

Release Velocity

Low Medium High

10 ft/s 10 < < 50 ft/s 50 ft/s

3 m/s 3 < < 15 m/s 15 m/s

A release with “High” speed flow has a greater hazard radius due to the misting transition. A mist, orspray, has usually a density lighter-than-air, that means that even at low wind speed the dispersion isquick and cover a wide area.

The second parameter adopted in this method is the substance volatility. The volatility of the mostcommon flammable liquids and flammable gases or vapors, are present into “NFPA 30: Flammableand combustible liquid Code”. The process groups all hazardous substances into five different“volatility categories”.



Category G: Flammable substances handled and processed as Gases and vapors.

Category 1: Materials, including LPGs, with a vapor pressure, at the operating temperature, above70 psia (0.483 MPa). Those material, once released, vaporize in a very short time, even if processed

liquefied.

Category 2: Materials are all Class 1A Flammable liquids with a vapor pressure of or less than 70psia and all other flammable and combustible liquids with a vapor pressure between 14.7 psia and 70psia at operating conditions.

Category 3: Materials are all Class 1B Flammable liquids with a vapor pressure of or less than 14.7

psia and all other flammable and combustible liquids with a vapor pressure less than 14.7 psia whenthe operating conditions are above their own flash point.

Category 4: Materials of Class II and heavier materials that are operated below their flash point

For further information about the volatility, and flash point refer to NFPA 30, 325 and 497

Once that the material category has been found, its hazard radius is only function of the materialrelease rate and the dispersion rate of gases and vapors.

Application to Non-Enclosed adequately ventilated locations containing Heavier-than-air gases orvapor:

The extent of the hazard radius, for each potential source, can be estimated by the following matrix. Thosemethod, based on API RP experience, is usually adopted for those type of source usually not revised by thestandards and codes.

For each substance category (G – 4), and for a specific rate of release, the radius can be estimated. Theapplication of those radius shall be done with the following precautions. Where the extent of an hazard radius

has been indentified, it must be revised by the good engineering safety judgment. The presence of physicalbarrier, inside the hazard radius estimated, shall be considered to modify the area extent. In many particularcases, the dispersion, especially for mixtures, must be evaluated using suitable software or models. In thosecases, which the dispersion isn’t subject at any external influence (e.g. Wind, Internal ventilation, etc.) thecloud dispersion is to be considered symmetric and uniformly.

Hazard Radius

Category 15-25 25-50 50-100

Category 23-5 5-25 25-50

Category 3



3 3-5 5-25

Category 4n.a. n.a. n.a.

Low

< 10 Gal(US)/min

Medium

10 – 50 Gal(US)/min

High

50 – 100 Gal(US)/min

Mass Release

For example, a category 2 fluid, with a mass release of 13 gal(US)/min (0.82 litri/s), the hazardradius is estimated within the range of 5-25 ft (1.5 – 7.6 m), with an adequate knowledge of thesource, and its area, the specific value can be estimated.

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